This invention relates to a method for fabricating composite materials. More particularly, the invention relates to a method for fabricating a curved composite ultrasound transducer. The method of fabrication permits the fabrication of curved ultrasonic transducers in large sizes. Such transducers are particularly useful in medical ultrasound diagnostic apparatus.
Transducers for ultrasonic diagnostic equipment are commonly fabricated from a single block of the piezoelectric ceramic lead zirconate titanate (PZT), however, other ceramics such as lead titanate and lead metaniobiate and certain polymers such as PVDV may also be used. Recently, composite piezoelectric materials constructed from a matrix of PZT rods disposed in a polymer have been developed. Such composite transducers have provided greater freedom of design in transducers. The term "composite transducer" as used herein describes a transducer which includes regions of an electrically active material (i.e. a piezoelectric material) which are embedded in a matrix of a second material. Preferentially, the second material is an electrically passive material (i.e. an insulator). The second material may have acoustic properties which are different from the acoustic properties of the active material.
Many composite transducers are manufactured in the form of flat discs. However, for certain applications in medical diagnostic ultrasound a curved focused transducer is desirable. However, the fabrication of a curved composite transducer has presented difficulties. One attempt to overcome these difficulties has been to fabricate the composite transducers from a polymer, such as polyurethane, which is extremely flexible, see e.g. the polyurethane composite transducers described in "Ultrasonic Probe Using Composite Piezoelectric Materials" IEEE Ultrasonic Symposium 1985 at pages 634-636 and "Medical Ultrasonic Probe Using PZT/Polymer Composite" described at Vol. 3 of the papers presented at the second "U.S./Japan Seminar on Dielectric and Piezoelectric Ceramics", Nov. 4-7, 1984. However, ultrasonic transducers fabricated from flexible polyurethane materials have been found to be less completely satisfactory.
Another method of forming a curved composite transducer is discussed in "1985IEEE Transactions on Sonics and Ultrasonics", Vol. SU-32 at page 499-513, in an article entitled "Piezoelectric Composite Materials for Ultrasonic Transducer Applications. Part II: Evaluation of Ultrasonic Medical Applications." In this article a composite transducer was manufactured from a relatively low viscosity epoxy which was molded into a curved shape by heating the already cured epoxy to a point above its glass transition temperature until it softened and bent. However, this approach has been found less than completely satisfactory, because large diameter transducers cannot be made due to the high shrinkage of the low viscosity epoxy resin. As the effect of shrinkage is proportional to the size of the transducer, the shrinkage builds up in a larger diameter transducers rendering them unusable. Higher viscosity epoxy is not usable in this method because it is thermosetting and will not deform under heat.
Another method for forming a curved composite ultrasonic transducers has been to grind concave and convex surfaces onto a flat composite transducer. However, only relatively shallow radius transducers are capable of being made by this method as the thickness of the composite block is limited by the depth of the groove that can be cut in order to produce the block by the dicing and filling method. PZT rods can be cut only to relatively short lengths and thus the amount of curvature achievable by grinding curved surfaces is quite limited. Furthermore, PZT/polymer composites can only be made in relatively thin sections.
The present invention is directed to overcoming these problems and provides curved composite epoxy/PZT transducers of large size having excellent electromechanical properties.